Creutzfeldt-Jakob disease (CJD) and Alzheimer's disease (AD) share many clinical and neuropathological features, and numerous instances are reported of both diseases occurring in the same family and even in the same individual. At the level of molecular biology and genetics, members of two or three CJD families with different PrP mutations have been shown to deposit pathological amounts of both the PrP and A4 proteins. We have studied an AD family with CJD features, in which an AD-causing mutation is present in the presenilin 1 gene on chromosome 14, the PrP gene is normal, and both PrP and A4 protein plaques are present in the brain. This is the first family in which an AD mutation has been shown to be responsible for the co-deposition of the two amyloid proteins, and emphasized the possibility of an intersection in the abnormal metabolic pathways leading to amyloid deposition. If so, therapeutic strategies valid for CJD may be applicable to AD, and at the moment only CJD is available as an experimental model for therapeutic testing; in particular, the use of capuchin monkeys, which have a comparatively long duration of illness, would be especially suitable as a model most closely approximating the human disease.